Current limiting fuse

ABSTRACT

Cartridge type current limiting fuses contain a fusible element within an appropriate cartridge connected to electrically conducting terminals at respective ends thereof. The fusible element is surrounded by a packed particulate mass of high resistivity high dielectric strength arc-constricting media which surrounds the fusible elements and is bound into a selfsupporting rigid but porous mass by an inorganic binder which maintains good electrical and mechanical characteristics at temperatures to which the bound particulate mass is subjected to during operation of the fuse to interrupt an electrical current.

United States Patent Frind et a1.

[ CURRENT LIMITING FUSE [75] Inventors: Gerhard Frind, Albany; Michael Owen, Latham, both of N.Y.; Ben Lee Damsky, Philadelphia, Pa.

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Jan. 29, 1973 [21] Appl. No.: 327,744

[52] US. Cl. .1 337/276, 337/273 [51] Int. Cl. H01h 85/18 [58] Field of Search 337/273, 276, 158, 246, 337/293; 200/144 C [56] References Cited UNITED STATES PATENTS 816,270 3/1906 Steward 337/276 UX 816,271 3/1906 Steward 337/276 UX 905,503 12/1908 Cook 337/246 1,213,777 1/1917 2,223,959 12/1940 Lohausen 337/158 2,740,187 4/1956 Jacobs et al. 337/276 X 2,772,334 11/1956 Latour 200/144 C 2,892,060 6/1959 Gaskill 337/158 X 3,166,656 1/1965 Hollmann 337/276 FOREIGN PATENTS OR APPLICATIONS 1,553,672 12/1968 France 1, 200 144 0 20,483 9 1902 Great Britain 337/227 Primary Examiner.l. D. Miller Assistant Examiner-Fred E. Bell Attorney, Agent, or Firm-Jerome C. Squillaro; Joseph T. Cohen [57] ABSTRACT Cartridge type current limiting fuses contain a fusible element within an appropriate cartridge connected to electrically conducting terminals at respective ends thereof. The fusible element is surrounded by a packed particulate mass of high resistivity high dielectric strength arc-constricting media which surrounds the fusible elements and is bound into a self-' supporting rigid but porous mass by an inorganic binder which maintains good electrical and mechanical characteristics at temperatures to which the bound particulate mass is subjected to during operation of the fuse to interrupt an electrical current.

10 Claims, 4 Drawing Figures CURRENT LIMITING FUSE This invention relates to current limiting fuses and, more particularly, to such type fuses as include a fusible element suspended within an insulating cartridge between a pair of electrically conducting terminals and surrounded by a mass of particulate matter which constrains an electric arc struck upon melting of the fusible element to thereby attain a current limiting circuit interrupting characteristic.

Current limiting fuses are generally utilized to pro tect electrical apparatus and circuits from fault currents in such a manner that the sudden interruption of the circuit current does not cause a high voltage transient which can be extremely detrimental to electrical apparatus, insulation, and the like. Such fuses are utilized in applications in which economy is an important factor and the replacement of a fuse within a reasonable time is an acceptable practice, as opposed to circumstances in which immediate reestablishment of the circuit is required, in which case circuit breakers or reclosers are utilized.

One of the most generally used type of current limiting fuse is the so-called particulate porous matter filled cartridge fuse in which a conductive fusible element or link is enclosed in an insulating cartridge between two terminals and surrounded by a mass of porous particulate matter such as sand or silica in finely divided form.

Such a fuse operates by the fusion or melting of the fusible link and the striking of a current carrying are between the unmelted portions of the link. Generally, as the arc burns, the unmelted portions become further separated as more of the link melts. Current limiting characteristics are obtained by the nature of the porous filler material which closely constricts the operating are but yet allows conducting specie of the arc to diffuse into the pores thereof and condense upon the relatively cool particles, thus removing conducting species from the arc while simultaneously confining and constricting the arc to essentially that volume originally occupied by the fusible link. Of great importance to the current limiting characteristic of such fuses is its characteristic of developing across the high resistance are, an arc voltage which is substantially higher than the system voltage. Conservation of energy considerations result in the development within the system of a voltage equivalent to the oversystem voltage and of such a polarity such as to rapidly force the arc current to zero, extinguishing the arc and interrupting the system current.

Many problems exist with respect to current limiting fuses having high voltage gradient arc characteristics. One of the greatest problems is the difficulty of obtaining uniform electrical characteristics, such as time to extinction of the arc and complete opening of the circuit for fuses of identical physical construction. It is also difficult to reproducibly manufacture such fuses with the same arc voltage gradient and current-time characteristic during arcing.

Another problem associated with fuses of the type described is the size of the fuse required to interrupt a given current. This is due in part to the low thermal conductivity of silica and other materials used as fillers therein. Since the effectiveness of the filler material as an arc constricting and current limiting agent is largely dependent upon its ability to remove thermal energy from the arc struck upon fuse operation, the low thermal conductivity of such fillers may be a disadvantage. On the other hand, since thermal conductivity and electrical conductivity are often closely related, and the filler must be a good insulator with a high dielectric strength, this problem has not been easily solved.

Accordingly, one object of the present invention is to provide current limiting fuses of the type described having improved current interrupting characteristics.

Still another object of the invention is to provide such fuses in which the electrical characteristics thereof are reproducible and consistent in normal manufacturing processes.

Another object of the invention is to provide such fuses with improved arc constriction characteristics.

Another object of the invention is to provide improved current limiting fuses of the type described adapted for use in DC. circuits.

Yet another object of the invention is to provide current limiting fuses of the type described having higher are voltage gradient and higher steady state current carrying characteristics.

A further object of the invention is to provide current limiting fuses which are able to interrupt electrical currents with a minimum dissipation of energy.

A further object of the invention is to provide current limiting fuses which upon operation allow a minimum amount of energy to be imposed upon associated apparatus.

Still another object of the invention is to provide improved methods of manufacturing current limiting fuses of the insulating particulate matter filled cartridge type,

Briefly stated, in accord with one embodiment of the invention, improved current limiting fuses of the particulate matter filled cartridge type include an insulating cartridge and a pair of conducting terminals at opposed ends thereof. An electrically conductive fusible element is suspended between the opposed terminals and is surrounded by a densely packed porous mass of fine particulate material such as a refractory oxide or nitride having a high dielectric strength and excellent insulating characteristics. The particulate mass is bound together into a self sustaining rigid mass by an inorganic binder which covers essentially all the surface of each particle of the mass, but does not substantially affect the porosity of the mass. The binder material is one which does not degrade the insulating characteristics of the particulate mass nor lose its mechanical binding properties at temperatures as high as the temperature of the particulate mass when subjected to a current interrupting are. In a preferred embodiment, the binder is the evaporation residue of a colloidal suspension of a refractory oxide in an evaporable solvent such as, for example, colloidal silica.

The novel features characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following detailed description taken in connection with the attached drawings in which:

FIG. 1 is a schematic cross-sectional view of a simplifled current limiting fuse constructed in accord with the invention;

FIG. 2 is a vertical view with parts broken away of a specific fuse constructed in accord with the invention;

FIG. 3 is a vertical cross-sectional view of the fuse of FIG. 2 taken along lines 2-2; and

FIG. 4 is a representative graphical plot of voltage and current as a function of time for a typical prior art current limiting fuse and a comparable fuse constructed in accord with the invention.

As is set forth hereinbefore, while the exact physical mechanism of operation of current limiting fuses utilizing a particulate mass of arc constricting material surrounding the fusible element is not precisely under stood, it is generally accepted that upon exceeding the melting point temperature of the fusible element, a portion thereof becomes molten and an arc is established between the immediately-adjacent portions thereof. As this are burns, the immediately adjacent portions of the fusible element become molten and the length of the are rapidly expands so as to encompass a large portion, if not substantially all, of the length of the fusible element. Since the space available to the arc is essentially, in theory, only that portion of the cartridge occupied by the fusible element, the arc is highly constrained and has a relatively high voltage gradient of the order of 200 volts per centimeter. Accordingly, the voltage rating of any given fuse is determined by the length of the fusible element. For low voltage fuses, a short straight fusible element, which may for example be in the form of a wire or a thin ribbon, may extend through the length of the fuse cartridge. For higher voltage rated fuses, the fusible element may be wound in a helix or other similar coiled, convoluted or serpentine configuration in order to obtain a greater length of fusible element and hence a longer arc within a given fuse cartridge. As the length of the fuse which is molten becomes greater, the arc voltage inserted in the circuit rises rapidly but continuously so as to avoid the creation of an instantaneous high resistance which could cause the creation of transient voltage peaks which are highly detrimental to circuits and apparatus which are associated with the fuse, particularly such circuits and apparatus containing inductive elements. This insertion of progressively higher resistance by rapidly expanding the length of the current carrying arc is in essence the mechanism by which the current limiting characteristics of the fuse are obtained.

Extinction of the current interrupting arc is accomplished by the constriction of the arc by the are constricting media which causes the development of a high voltage across the arc which induces a matching and opposing high voltage within the system with the affect of the opposing voltage forcing the arc current to zero. Thus, such fuses are effective to interrupt D.C. currents and do not depend upon a naturally occurring zero value of current as in AC circuits. During arcing, the porous nature of the arc constricting matrix which constrains the arc allows for the dissipation of conducting species into the voids thereof and condensation upon the particles, helping to dissipate heat and limiting the maximum force applied to the exterior fuse cartridge.

On the other hand, in an alternating current circuit, extinction may occur by virtue of the combination of the aforementioned effects and the occurrence of a a zero value of current or an approach to the zero value fusible element. Normally, however, even in A.C. circuits, the arc current is forced to zero before the occurrence of a normal current zero value, as in the DC. mode of operation. Since on A.C., the voltage tends to decrease from a peak value in the latter half of each half cycle, extinction may be easier than on DC.

The foregoing described mechanism of current limiting fuses of the packed particulate arc quenching type is not always achieved in practice. One significant reason for a failure to achieve the ideal mode of operation lies in the porous nature of the particulate matter and the possibility of inhomogeneous distribution thereof with the inclusion of voids or less densely packed portions of particulate material. In addition to the foregoing, a loose agglomeration of particulate particles surrounding a fusible element is susceptible to physical displacement by the intense pressure of the confined arc during operation of the fuse. Should any of the foregoing situations exist, it is then possible for the current carrying arc to expand into an already existing void, or a void created by expansion of the are due to its high pressure and the mobility of the particles of the surrounding medium. Such expansion then lowers the constraint upon the arc and lowers the voltage gradient thereof so that the arc may continue to burn for a period of time which is unacceptable for proper protection of the associated equipment. At the ultimate limit, should the arc burn for an extended period of time, the surrounded particulate matter may become deeply molten and become electrically conductive, leading to failure of the fuse.

Prior workers in the art have attempted to solve this problem by causing the particulate matter surrounding the fusible element to be as densely packed as is possible. Thus, for example, as disclosed in US. Pat. No. 2,740,187 issued Apr. 3, 1956, a cartridge including a fusible element is filled with particulate arc-quenching material, while the cartridge is vibrated upon a vibrating table, thus causing a highly dense and closely packed filler which is less susceptible to the aforementioned failure mechanisms. Despite the foregoing, fuses prepared by such vibration do not provide sufficiently guaranteed operational performance so as totally to avoid failure due to the aforementioned failure mechanisms. Additionally, and of greater importance, even mechanically compacted pulverent or particulate matter type cartridges can become erratic in their behavior. That is to say, a number of fuses prepared by the same process and having the same physical characteristics may not exhibit the same characteristics, such as are current, time of total interruption of the arc, and arc voltage gradient, and PT let through term", which is the integral of l dt, where I is the current during arcing during the time interval from fuse melting to are extinction.

The failure of prior art fuses to exhibit uniform electrical characteristics is of particular importance because, as may readily be appreciated, as the current rating of a particular fuse exceeds a predetermined value, it becomes necessary to protect a given circuit or apparatus with a plurality of fuses in parallel. This is because there is a maximum value of cross-sectional area of the circuit interrupting arc which may be extinguished by the mechanism of the particulate matter type cartridge fuse. Empirically, it has been determined that for a round wire, a diameter of approximately 0.010 inch is the maximum diameter which may usually be utilized in fuses of this type, and with a rectangular cross sectional ribbon, a ribbon having thickness dimensions (the lesser of the width and thickness) of approximately 0.010 inch, is approximately the maximum size element which may usually be utilized. The reason for this limitation is that, should the cross-sectional dimension of the fusible element which determines the cross sectional area of the constricted arc during fuse operation, exceed this value, the voltage gradient of the arc may become insufficiently high to permit extinction thereof in a time sufficient to achieve satisfactory fuse operation.

Experience has shown, however, that it is difficult to construct fuses with a plurality of parallel fusible elements in a particulate matter arc-quenching matrix without having a single one of the plurality of fusible elements, or the are created thereby, carry a disproportionate amount of current, thus causing failure of the fuse to operate as desired. In such arrangements, substantial equal sharing of the total current load by each of the plurality of parallel fusible elements, or the arcs struck thereby, is essential in order to avoid failure of the fuse by a lesser number of the total number of arcing paths conducting a prohibitively high value of current with the result that either the arc cannot be extinguished or excessive energy is admitted into the circuit. The difficulty in obtaining reproducible characteristics of fuses of this type, even with mechanical compaction during processing, has rendered the use of multiple fusible elements in parallel for higher current operation difficult.

In accord with the invention, we have found that greatly improved electrical characteristics are obtained from fuses of the particulate matter arc-constricting material, current-limiting, cartridge fuses when the particulate matter is both mechanically compacted and tightly bound. into a rigid, self-sustaining, but still porous mass by a suitableinorganic binder, which neither degrades the electrical characteristics of the arcconstricting material nor loses its mechanical binding characteristics at the temperature to which the porous mass of arc-quenching material is subjected during operation of the fuse.

Fuses constructed in accord with the present invention possess dramatically improved characteristics as compared with fuses of the prior art, as described hereinbefore. Among these improvements are a significant increase in the arc voltage gradient, by as much as 50 percent due to greater constriction of the currentcarrying arc; a much lower current during fuse operation, than fuses of the prior art; a decrease of the socalled let-through term (often referred to as I T letthrough, which, when multiplied by resistance, is a measure of energy) to the protected circuit elements by as much as 1/5 of that of fuses of the prior art; reduction of the total energy dissipation during operation by as much as 50 percent from that of comparable fuses of the prior art; reliable reproducibility of electrical characteristics to the extent that fuses utilizing a plurality of identical fusible elements in parallel operate with equal sharing of the total current amoung the arcs struck by the fusion of individual fusible elements during operation; and a greatly increased thermal conductivity of the porous matrix, resulting in a significant increase in steady-state current carrying ability of a given size fusible element.

In FIG. I of the drawing, a typical fuse constructed in accord with one embodiment of the present inven' tion, includes an insulating fuse cartridge l terminated at either end by a conducting terminal 2, which terminals are connected by a fusible element or link 3 having a plurality of constrictions 4 therein and suspended between a pair of support members 5, the distance between which determines the maximum length of the current interrupting arc.

Cartridge ll of the fuse of FIG. ll may be fabricated from any suitable insulating high dielectric strength material suitable for the value of voltage for which the device is rated. Typically, such materials may be fiberglass, epoxy resin, quartz, or ceramics, for example. Terminals 2 are conveniently constructed of a conductive material as, for example, copper or aluminum, which may, for example, be silverplated to improve oxidation resistance thereof, support members 5 which are generally large in cross-sectional area as compared with the cross-sectional area of fusible element 3, and are generally fabricated from the same material as terminals 2. Fusible element 4 may be fabricated from a conductive material as, for example, silver, copper, silver-coated copper, aluminum, or silver-coated aluminum, tin and zinc. One or more constricted regions 4 may be provided along the length of fusible element 3 in order to cause the electrical resistivity thereof to be higher than the remaining portion of the fusible element to cause the initial melting of the fusible element at that point. In the instance of a relatively low voltage fuse, only a single or a few constricted portions may be provided and the arc is established thereat and burns back rapidly in both directions therefrom. In higher voltage applications, a large plurality of constricted regions are provided in the fusible element in order to cause substantially simultaneous establishment of a large plurality of arcs among which the voltage gradient is distributed, causing many burnings in order to facilitate the rapid establishment of a plurality of series connected arcs and a high voltage gradient between the fuse terminals in the shortest period of time consistent with the avoidance of the sudden insertion of a high resistance in the circuit, resulting in the creation of detrimental high voltage transients. The region within cartridge 2 not occupied by support members 5 and fusible element 3 is substantially filled with a tightly packed matrix of particulate material 6 having excellent insulating characteristics and a high dielectric strength and providing a porous matrix for the entrance of conducting specie from an established current-carrying arc to cause rapid energy removal therefrom, while still effecting constriction of the arc. Matrix 6 may be composed of any refractory high resistivity material having high dielectric strength, but preferably is composed of one of the oxides or nitrides of metals such as silicon, magnesium, aluminum, beryllium, calcium, and strontium, and mixtures of such compounds, for example. Silicon dioxide, having the highest electrical resistance of known insulators and being readily available as sand or quartz, is generally preferred for manufacturing purposes. Matrix 6 is as densely packed as is possible by such mechanical expedients suitable for manufacturing techniques as, for example, vibrating while pouring the matrix into the cartridge. Matrix 6 is also bound into a rigid self-supporting, tightly adherent body by the admixing therewith either prior to filling or subsequent to filling of a suitable inorganic binder which is applied in sufficient quantity as to coat each individual particle of the particulate mass over substantially the entire surface thereof without substantially diminishing the porosity of the matrix.

The electrical characteristics of the binder utilized to bind matrix 6 into a rigid self-supporting mass of porous particulate material are stringent and are satisfied by careful selection of the chemical and physical nature thereof. The binder material must be such as to avoid degrading the electrical characteristics of the porous particulate material to which it is added so that during operation of the fuse, when the bound matrix of porous particulate material is subjected to the high temperature of the current-carrying arc, which typically is at least of the order of 20,000K., the resistance characteristics of the matrix is not adversely affected by the presence of the binder. Additionally, the binder must be of such nature as to withstand such temperature conditions during arcing without either releasing a sufficient amount of gas as to adversely affect the mechanical integrity of the cartridge, nor lose its physical binding characteristics. Additionally, since the mode of operation of such fuses is such that a substantially elevated temperature of as high as approximately 400C may be required to be tolerated by the fuse for long periods of time without fusing, the binding must be such as to neither evolve substantial gaseous material therefrom, lose its binding characteristics, nor decompose to release any electrically conductive substance at lower temperatures of the order of 400C for extended periods of time, nor the higher temperatures of the currentcarrying arc for relatively short periods of time. Should any such material as, for example, a carbonaceous material, be liberated, such material would, during arcing degrade the insulating characteristics of the bound particulate arcconstricting material and render total extinction of the conduction current difficult, if not impossible.

In this respect, it is obvious that organic material as, for example, phenolic resins, epoxy resins, melamine resins, and silicone resins, are not satisfactory due to the presence of carbon which can be liberated either in the form of a carbonaceous-conducting residue or, in the presence of oxygen as carbon dioxide gas which could adversely affect the physical integrity of the fuse, together with the presence of other readily dissociable materials, clearly renders organic binders unsuitable for binding matrix 6. Within the category of inorganic binders, any material suitable as a binder which does not lose its binding characteristics at extended times as, for example, several hours at temperatures of up to 400C, or for relatively short periods of time as, for example, several milliseconds to several seconds at temperatures of the current-carrying arc as, for example, 20,000K, and which does not under such circumstances decompose or otherwise change in character so as to emit either a substantial quanity of gaseous material or electrically conductive matter such as carbon or graphite which could degrade the insulating integrity of the porous pulverent material constituting the binder, is suitable. Within the acceptable inorganic compounds, we find that, ideally, oxides and nitrides of the same materials as constitute the porous pulverent arcquenching substance, are ideally suited. Additionally, it is preferred that these materials be added in colloidal suspension to the porous pulverent material and treated to a suitable heating and drying cycle to evolve the volatile solvent therefrom. Silica sol, which is a colloidal suspension of silicon dioxide in water, and suitable colloidal suspensions of silicon dioxide, magnesium oxide, aluminum oxide, beryllium oxide, calcium oxide and strontium oxide, for example, in water, ethyl alcohol, methyl alcohol, acetone, glycols, or ether, for example, all of which evaporate leaving no residue to contaminate the porous particulate bound binder, are suitable. Preferably, we find that the solid constituent of the colloidal suspension should be within the range of approximately 25-40 percent by volume for optimum manufacturing processes.

The size of the particles which constitute the particulate arc-constricting media surrounding the fusible element of fuses of the invention is not exceptionally critical, however, the particles must be large enough to cause appropriate voids therein when closely packed and bound in accord with the invention so as to permit diffusion of arcing specie thereinto during the are extinction process. On the other hand, the particles should be sufficiently small as to form a densely packed agglomerate which is adequate to constrict the arc, since arc constriction by the arc constricting media is the means by which the high voltage gradient of arcs struck during operation of fuses in accord with the invention is achieved. This is because the voltage gradient of the arc is in a rough approximation inversely proportional to the diameter of the arc, or arcing space available. In general, the size of particles suitable is roughly of the order from 0.001 inch to 0.100 inch average diameter. There is no requirement, however, that all particles be of the same diameter and, as a matter of fact, with appropriate mix of particles within the range, more dense packing is possible.

In general, the volume of the cartridge of fuses in accord with the invention which is actually occupied by the particulate arc-constricting media, is approximately 60 to percent of the total volume, for example, depending upon particle size and distribution, as well as degree of packing, the remainder being voids into which the arcing specie may diffuse to cool the arc and aid the extinction thereof.

The foregoing may readily be determined by the fact that in manufacturing fuses in accord with the present invention by one method, given by way of example only, the cartridge is completed, including the fusible link therein, with appropriate apertures in either end, the lower aperture is closed, a colloidal suspension of colloidal silica, for example, is poured into the cartridge and the amount taken constitutes approximately 20 to 40 percent of the volume of the empty cartridge. The cartridge being filled with particulate matter, the binder is added to overflowing, the lower aperture is then opened and the binder is allowed to drain therefrom while the matrix is retained. This process may be repeated several times in order to assure that substantially all of the surface of all of the particles constituting the porous particulate arc-constricting matrix is covered with a thin film of binder. During such filling, in one example, a 20 cc. volume fuse cartridge is filled with sand and after packing the cartridge accepted 6.0 cc. of a colloidal suspension of colloidal silica, of which approximately 1.8 cc. is recovered when the binder is drained from the cartridge. After draining, the cartridge is air dried by passing slightly compressed air therethrough then heating for a sufficient time at an appropriate temperature to evaporate the volatile solvent which may, for example, be water or alcohol, in order to remove therefrom all constitutents other than the binder material originally in colloidal suspension. Tests indicate that the actual amount of binder present after such baking to remove the volatile solvent is of the order of 1 percent of the total weight of the bound arcconstricting medium. In accord with an alternative method for manufacturing of fuses in accord with the present invention, the fuse cartridge may be assembled with apertures, the lower aperture temporarily plugged, and the particulate pulverent material externally mixed with the colloidal suspension by pouring the suspension through the pulverent or particulate material in a separate container and draining the same, then pouring the wetted arc-constricting material into the fuse cartridge with or without vibration, but preferably with vibration to insure the densest possible packing of the porous particulate material. After filling of the cartridge, the lower aperture is similarly opened to allow any residual binder material to drain and a similar curing, as for example, by first air drying with a flow of compressed air at approximately 2 psig for 15 minutes then heating at 70C for one to three hours, then further heating at ap proximately 150C for one to twelve hours, is sufficient to completely evaporate the volatile solvent and leave the particulate material in a rigidly bound porous arcconstricting matrix. To facilitate curing of the matrix, it is important to air dry after draining and before heating to cause an initial air drying by blowing air under moderate pressure, i.e., two atmospheres, through the fuse cartridge for tens of minutes prior to baking. This greatly reduces the heating required. The final baking temperature should be in excess of approximately 100C and can be any temperature which is not high enough to adversely affect either the fuse cartridge or the fusible link contained therein. This temperature is, however, not critical provided it is within the functional range specified above. The foregoing description of process steps and numerical values of temperature, time, and other drying parameters, for example, are given by way of example, and are not required in all manufacturing processes.

In general, cartridge fuses of the type described herein in accord with the present invention are normally characterized by those skilled in the art as either low voltage or high voltage fuses. A low voltage fuse is generally considered to be any fuse with a voltage rating less than 1000 volts and may havea current rating of from 1 to 4000 amperes, for example. High voltage fuses, on the other hand, are generally considered to be those having voltages in excess of 1000 volts and extending as high, for example, as to a standard industry rating of 69 kV or higher. Such high voltage fuses may have current ratings of approximately 1 to 1000 amperes, for example. As is mentioned hereinbefore, the voltage rating of the fuse determines the length of the fusible element, since the arc voltage gradient is approximately 200 volts per centimeter and the length of the arcing path is normally dependent upon or establishes the voltage rating for the fuse. The current rating of the fuse is normally determined by, or controls, the thickness or cross-sectional area of the fusible element, the lower the current rating, the smaller the crosssectional area of the fusible element. As is set forth hereinbefore, however, the foregoing is true in the instance of a single conductive element only constituting the fusible link. In certain instances such as in order to secure low current interrupting characteristics and/or high currents which would require or cause a fusible element to have a cross-sectional dimension of the smallest such dimension of approximately 0.010 inch, a plurality of links in parallel are required, since an arc having a volume occupying the space of a fusible link, the smallest dimension of which is greater than this value cannot be quenched by the mechanism of fuses of this type. For current ratings which exceed this value (approximately 20-50 amperes depending upon fuse design) a plurality of fusible elements are utilized in parallel.

As is set forth hereinbefore, one of the greatest advantages of fuses constructed in accord with the present invention is the high degree of reliability and reproducibility of the electrical characteristics thereof. This advantage and this characteristic are of particular significance for high current fuses utilizing a plurality of fusible elements in parallel. Tests conducted with fuses in accord with the present invention, as compared with fuses utilizing unbound particulate arc-constricting matrices, clearly show that, whereas parallel connected fuses of the prior art do not share current equally, fuses constructed in accord with the present invention invariably have substantially identical currents flowing in substantially identical parallel fuse elements during operation. This characteristic is particularly of significance when the interior cross-sectional area of the fuse cartridge is large, as for example, over 1 inch inside diameter. In general, fuses constructed in accord with the present invention show the greatest advantages over fuses of the prior art in the ranges of cartridge diameter of from one to four inches, or greater. For fuse diameters smaller than one inch, the densely packed unbound arc-constricting media fuses seem to be adequate to interrupt in a current limiting mode the currents and voltages for which they are rated when constructed in a laboratory or test pilot run situation. On the other hand, since the achieving of the dense closely packed matrix is a difficult thing to do for mass production manufacturing processes, it is of equal advantage for commercial utilization, due to the great economic benefit of avoiding protracted vibration during filling, that the bound arc-constricting fuses of the present invention be made rather than unbound tightly mechanically packed arc-constricting media type fuses.

In general, the diameter of the cartridge for a given fuse is representative of the thermal energy dissipating characteristics required thereby. The greater the thermal energy which must be dissipated in a given fuse, the larger the diameter of the cartridge. Large diameter cartridges may be required either because the fusible element is at or close to the maximum cross-sectional area due to the high current rating of the fuse, or alternatively, because the fuse has a high voltage rating and- /or a plurality of fusible elements are contained therein or a single fusible element passes through the arcconstricting media in a convoluted, serpentine, helical, or other complicated configuration so as to obtain a long arc length within a given size fuse cartridge or to.

provide a plurality of parallel conduction paths. As a practical matter, fuse cartridges for production fuses do not generally exceed four inches in diameter because, as the diameter of the fuse cartridge increases, the difficulty of obtaining uniform packing of the particulate arc constricting material even with vibration becomes more difficult to obtain because of the large number of fusible elements contained therein.

The striking advantage of fuses constructed in accord with the present invention may be partially appreciated from the data presented in Table 1, below. Table I is a tabulation of maximum energy interruption tests run on respective pairs of cartridge fuses of the type described herein, of diameters of one, two, and four inches, connected in pairs, in parallel circuit relationship with silica sand as the particulate arc-constricting material and illustrates the success or failure of the fuse pair with respect to both unbound and bound sand arcconstricting media contained, and also illustrates whether or not the total current through the pair of fuses was equally shared by the two parallel connected fuses, whether the fuse array failed, or whether it did not. A failed fuse is one which does not extinguish the current upon the occurrence of a fault current. Such occurrence in a commercial installation would result in the destruction or serious damage to electrical apparatus or circuits designed to be protected by such fuses and is totally unacceptable.

TABLE 1 MAX.ENERGY CURRENT TESTS PARALLEL FUSES-ONE ELEMENT PER TUBE All fuses 8" in Length. Identical Fuse Link Dimensions Tube Tubes In Size Packing Which Took Test Parallel Inches of Matrix Binder Major Energy 9 A l Poor X B 1 10 A 2 Poor X Failed B 2 l l A 1 Good X B l 12 A 1 Good 3 B 4' Good X Failed 13 A 1 Good B l Poor X 26 A l X Equal Sharing l3 2 X 32 A 1 Good B 2 Good X Failed 34 A 1 Good X B 4 X 35 A l X Equal Sharing Tests On Fuses With Multiple Elements Per Tube, Parallel Tubes 41 A 4 X Equal Sharing 45 B 4 X 42 A 4 X Equal Sharing From an examination of Table 1, it is apparent that in Tests 9 and 10 in which equal diameter size fuses, having one fuse constructed in accord with the present invention with binder and one fuse constructed in accord with prior art in which the arc constricting media was poured into the cartridge without binder, resulted in an unequal distribution of current with the major part of the current and energy dissipation concentrated in the unbound fuse. In the 1 inch diameter fuse, although this did happen, the fuse did not fail, but current was not properly limited. In Test 10, the unbound fuse failed, rendering the parallel connection totally inoperative and any equipment connected therewith would be deleteriously affected. In Test 11, a pair of 1 inch diameter fuses in one of which the matrix was tightly packed by vibration and in the other was packed and bound in accord with the present invention, the major part of the energy and current was dissipated in the unbound fuse, but the fuse aggregation did not fall though current was not limited properly. In Test 12, a 1 inch diameter unbound vibration packed and a 4 inch diameter unbound vibration packed fuse were tested and the major portion of the current and energy was dissipated in the larger fuse which failed, rendering the parallel connection of fuses insufficient for the purpose to which they are intended. In Test 13, two 1 inch diameter cartridge fuses, one of which was loosely packed, the other of which was vibration packed, neither of which was bound in accord with the present invention, operated and the major portion of the current and energy was dissipated through the poorly packed cartridge which did not fail but which did not limit current properly. In Test 26, a 1 inch and a 2 inch diameter fuse, both using a bound sand pack matrix, in accord with the present invention, shared the current and energy dissipation equally and limited current satisfactorily. In Test 32, 1 inch and 2 inch diameter cartridge fuses having unbound tightly packed sand showed an unequal distribution of current and energy with the major portion thereof in the larger diameter cartridge, which failed. In Test 34, a 1 inch diameter fuse having a tightly packed but unbound matrix, and a 4 inch diameter fuse having a bound matrix, were connected in parallel and the major portionof the energy was dissipated in the 1 inch diameter unbound fuse cartridge, which did not fail but did not limit current properly. In Test 35, similar 1 inch and 4 inch diameter fuses were utilized having bound matrices in accord with the present invention, and exhibited equal current sharing. In Tests 41 and 42, a pair of 4 inch diameter fuses, both of which used a bound matrix in accord with the present invention, were connected in parallel. In both tests, both sets of fuses exhibited equal sharing of current.

A comparison of the various tests shows, for example, that with fuses in accord with the present invention, the bound fuse always showed a higher arc voltage gradient than a comparable fuse of identical geometry using unbound arc-constricting matrix, resulting in the major portion of the current going to the prior art type unbound matrix fuse. None of the fuses in accord with the present invention ever failed at the voltage of the test and approximately 50 percent of the prior art fuses failed, i.e., did not interrupt. When two fuses constructed in accord with the present invention, having different diameter cartridges were connected in parallel, the fuses equally shared the load despite the difference in diameter. On the other hand, when fuses of the prior art of different diameters were connected in parallel, the larger diameter fuses, all others being equal, invariably carried the larger portion of the load and in Tests 12 and 32, failed.

From the foregoing Table, it is apparent that fuses constructed in accord with the present invention clearly exhibit a higher are gradient than prior art fuses and clearly show the reproducibility effect even with different diameter fuse cartridges, so that equal sharing of load in parallel connected fuses, i.e, high current fuses, is no problem and may readily be achieved, whereas, the tested fuses constructed in accord with the prior art exhibit unequal current sharing, even with identical diameter cartridges and identical characteristics of the arc-constricting medium.

Another advantage of fuses in accord with the present invention, as compared with fuses of the prior art, is the rapidity or speed with which the fusible element under conditions of low fault currents is consumed by the lengthening are when the fuse first operates by the melting of the constricted section of the fuse link at which an initial melting occurs. To those skilled in the art, the rate is characterized as the burn-back" rate.

From Table 2 it may b e seen that. for the straight length unbound arc-constricting media tvpe fuse of Type 1, burn-back rates vary from as low as l l centimeters per second at 22 amperes to approximately 40 centimeters per second at 50 amperes. Type 2 fuses ThlS chiltflcteflshc 1S OfImPOTtimCC Particularly h htgh using an insulating core and an unbound matrix exhibit Voltage tusefi; bechusc h htgheh the h Tate burn-back rates of from 15.5 to 31.6 centimeters per the more tuptd a hlgh reslsttlhce ot the g Voltage g second for currents of 22 to 39 amperes. Type 3 fuses dient arc isir ser ted into the circuit to achievesmoot Utilizing a straight fusible length and a bound matrix in current limiting interruption characteristics. ll the rate accord with the invention exhibit burn-back rates of of burn-backis not sufficiently rapid, the fuse can fail. f 145 centimeters per Second at 22 umperes to 55 While the failure mechanism is not fully understood, centimeters per second at SOamperes. Type 4 fuses utione postulated theory has to do with the formation of Zmg a ribbon wound about a core and a bound area f d mass f that portion f the particulate or constncting media in accord with the invention exhibit m s r c ns ri ti di i di t l dj t h burn-back rates of 23 centimeters per second at 22 amfusibie element s h f d masses are generally d 5 peres to 43.3 centimeters per second for 39 amperes. nominated as fulgurites and are normally highly re- Thus. by comparing from Table 2 the data of fuses of sistive. While fulgurites are invariably formed upon op- YP S l and hich are the same except for binding ermion of fuses of the type described, should the arc of the matrix in accord with the present invention. it is bum f an excessive iength f time due to a i b 7 apparent that the burn-back rate increases by in excess back rate and a low rate of voltage increase, the exces- Qt PQ t tUSCS Constructed in accord with th sive melting of the arc-constricting material may form pt h hh with hound arc-constricting an electrically conductive fulgurite which may result in s fi ttw y mparing fuses of Type 2 and Type fuse failure. The formation of a conducting fulgurite or 4 h h httcr h y g "1' hound onstricting anolhcr Conductive equivalent can be avoided by Cau$ 7 media in accord with the present invention. shows an ing the fuse link to burn back rapidlyand establish a P Y F huth'btlck h of th excess of 30 P high resistance are before the burning time ofthe initial tor {tccord Wtth the Prebeht lnventlon high-temperature arc can cause sufficient fusion of the htubes In accord with the Prefient n lOn arc constricting material to form a conducting fulguthe greatly P P F P tuses respect to the rite. While this theory may not be correct, it is a well- 2 bumbach charactensth? of fuse thdichtthg documented fact that a low burn-back rate often results f rffduceft P bi y f il re of the fuse in in the failure of the mm I which-da tulgurite may be formed. or a similar mecha- In Table 2. data are presented which illustrate the ay) n g a f rate burn-back rate for numerous fuses. having different but Volt, 6 I g g carmqge fuses for 9 quite similar cross-sectional areas of the fusible link g 33: 3 gg pnor Thus p and in which the current varies from 22 to 50 amperes sued 5 i h 3J3I769O and compares the burn-back rate for four type fuses. which a p'mmus f i -F .fuse m Fuses of Type I are fuses having a straight link with an o the E e ft uuhzed as unbound packed particulate silica sand arc-constricting T d P e e d t i unspec' 7 I I 11e inder. Similarly, a later-filed patent issuing to the media. Tvpe fuses have a non-conducting core. about 0 I I .ame Hollmann et al., US. Pat, N 3 16 5 refers which a ribbon of fusible material is wound to attain a t th 1 o e app ications resulting in the aforementioned pahigher voltage rating and is surrounded by a matrix of t d t I I I en s an specifies that suitable binders or solidifiers tightlv packed unbound silica sand. Type 3 Fuses lune are me amine resins, silicone resins, and calcium sula single fusible length and a bound silica sand arcu Th t v I a e. e eac ings of the Hollmann et al. patents do constricting media in accord with the invention, and Q t l h 4 no resu t int eadvantages set forth for fuses in accord Tvpe 4 tuses have a core about which a tuse ribbon is ti i 1 1e present invention. Initially, it should be noted wound and is surrounded by a bound silica sand,arcthat the Hollmann fuses contain an 6 0x constricting media in accord with the invention. p y Sm TABLE 2 Test Test Burn-Back Fuse Test kV Amp Element Core Binder CM/See Type 46 do. do. do. I

51 do. 39 do. i

61 do. 43 0.0056 x 0.187 1 63 do. do. do. l

58 do. 50 do. 40 1 41; do. 22 0.0043 x 0.187 x 15.5 2

53 do. 39 do. X 2

55 do. do. do. X 31.6 2

45 do. 22 do. X 14.5 3

47 do. do. do. X 3

52 do. 39 do. X 3

62 do. 43 0.0056 x 0.187 X 3 64 do. do. do. X 3

59 do. do. X 55 3 49 do. 22 0.0043 x 0.187 X X 23 4 54 do. 39 do. X X 4 56 do. do. do. s x 43.3 4

tainer or cartridge which, in the only relevant embodiment in which a binder is used, is formed by spinning a quantity of fluid epoxy resin in a mold about a pair of orthogonal axes. In this embodiment, the patentees first form the fusible element surrounded by a bound filler and use the binder solely for the purpose of forming a solidified mass about which the epoxy resin cartridge is formed. While no particular binders are set forth in the first-filed patents, the later filed patent does set forth suitable binders which in accord with the teachings of this invention are clearly unsuitable since they fail to satisfy the criteria of not forming or releasing conductive material, such as carbonaceous matter, at the temperatures of operation of the fuse, and/or do not maintain their physical integrity so as to continue as a binder at such temperatures. Thus, for example, typical melamine resins as listed in the scientific literature soften and lose their binding characteristics at a temperature of approximately 400F (210C). (Amino Acids, by John F. Blais Reinhold Pub. Corp., New York). Similarly, typical silicone resins suffer the same softening and loss of binder characteristics at approximately 500F (260C). (High Temperature Plastics by Brenner, Lum, and Riley, Reinhold Publishing Corp., New York, 1962). In addition to the foregoing, both silicone and melamine resins decompose at temperatures of from 400 to l000C. If such decomposition is conducted in air, the melamine resin, for example, merely loses its binder characteristics at approximately 400 to 600C and releases substantial quantities of hitrous oxides and carbon dioxide, which can cause explosion of the fuse. If, on the other hand, the heating is conducted in the absence of air, the melamine resin carborizes, releasing a carbonaceous material which can severely affect the recovery strength characteristics and degrade the insulating properties of the refractory insulating material of which the arc-quenching material is formed. Calcium sulfate, another material which is set forth as a suitable binder material in the Hollmann patents, likewise suffers a loss of binder strength. Thus, as reported in US. Bureau of Mines Technical Paper 625, page 3, published in 1941, in an article by Kelly, Southard and Anderson, report that at temperatures from 900 to 1200C, depending upon ambient pressure, calcium sulfate decomposes to form calcium oxide and S0 Since the arcing temperature of the current interrupting arc of fuses of the cartridge type current-limiting fuse is known to be in the region of 20,000K, and the temperature of the arcconstricting media surrounding the arc is of the order of several thousand degrees C, it is clear that all of the binders set forth as suitable for the I-Iollmann cartridge fuses do not satisfy the criteria of the binders utilized in accord with the present invention since they lose binding characteristics at the temperature of the current carrying arc during fuse operation.

The foregoing, however, is consistent with the teachings of the I-Iollmann patent in that the binder is utilized, not to improve the electrical characteristics of the fuse, but as a solidifier of the matrix to help in manufacturing processes for ready facilitation of a shell to be formed thereover. This is as distinguished from the purpose of the binder of the present invention, wherein the binder is utilized to render the particles of the arcconstricting media immobile. Such immobility. is essential, particularly in high voltage fuses, where the intense pressure generated by the highly constrained arc tends to move the arc-constricting media particles and form voids, lowering the arc voltage gradient. Such problems are not present in fuses of the l-Iollmann et al type, which are essentially low voltage fuses in which the arc pressures are substantially less than the arc pressures to be found in high voltage fuses in which a principal applicability of the present invention lies.

A specific current-limiting fuse constructed in accord with the present invention and utilizing a cartridge type configuration with a plurality of fusible elements in parallel therein surrounded by a bound particulate mass of silica sand, is illustrated in FIGS. 2 and 3 of the drawing. FIG. 2 is a vertical plan view with parts broken away, and FIG. 3 is a horizontal section taken along lines 33 thereof. The particular fuse illustrated in FIGS. 2 and 3 is a 700 ampere motor-starter fuse rated for 5 kilovolt maximum voltage. It is used as a backup fuse designed to operate only upon a substantially dead short but to endure short lived transient currents up to 200 percent maximum rated amperage without melting. This is typical of backup fuses in that other fusible elements or switches within the circuit are designed to interrupt for moderate overloads, but are incapable of sustaining full fault current of a dead-short, or substantial dead-short without failing. In FIG. 2, fuse 10 comprises a pair of individual insulating fuse cartridges 11 and 12 interconnected with a pair of massive buss connectors l3 and 14 at opposite ends thereof. Each of fuses 11 and 12 comprises an insulating cartridge 15 which may, for example, be made of epoxy or phenolic resin, fiberglass, or a ceramic and are approximately 18 inches long and have an outside diameter of approximately 3.5 inches. The wall thickness thereof may conveniently be approximately l/ 16 inch and the structure thereof is preferably an epoxy fiberglass construction. Interior of cartridge 15, a pair of inverted fluted conductive cups of silver-plated copper of approximately A; inch thickness are inserted into the cartridge with approximately eight legs, 20, extending outwardly and affixed to a conductive endcap 17 which in turn is connected to buss 14. Between and centrally located of cylinders 16, an irregular insulating core which may, for example, be in the shape of a fluted cylinder, as illustrated, lends physical and mechanical stability to the structure. Such insulator may be constructed of a ceramic material, for example. A plurality of fusible elements 19 extend between fluted cylinders 16 and extend through slotted apertures therein and are electrically connected, as for example by spot-welding to the exterior side thereof to form excellent electrical contact thereto. As is illustrated in FIG. 3, fusible elements 19 are disposed substantially equidistant to one another and are disposed between insulating core 18 on one hand and the interior wall of insulating cylinder 15. Twenty-two such fusible elements, each having a thickness of 0.004 inch and a width of 0.277 inch, each of which contains 30 holes of 0.188 inch diameter on /& inch centers along the length thereof, which is approximately 15 inches, to provide a plurality of constricted areas for simultaneous fusion upon operation of the fuse. The physical structure of each cartridge is assem bled and a /8 inch pipe tap is threaded in the endwall member of each of fluted members 16 and high purity, particulate consisting essentially of 40 mesh silica is poured into the upper aperture while the cylinder is held in a vertical position, while the lower aperture is closed with a tap. During pouring, the cylinder is mechanically vibrated for approximately 15 minutes. This vibration facilitates an increase in the quantity of silica sand which may be taken therein of approximately 15 percent, although depending upon time and intensity of vibration, increases of from to 25 percent may be achieved and are satisfactory. After filling and vibrating to form a compacted mass, the compacted mass is filled with a 35 percent by weight colloidal suspension of suspended silica in water. A suitable colloidal silica may be obtained from Nalco Chemical Company, Metal Industries Division, Chicago, Illinois 60717, and is identified as NALCOAG 1034APH3, containing approximately 33 percent silicon dioxide, 67 percent water, 0 percent alcohol, and less than 0.005 percent sodium oxide. After filling with the colloidal silica, the lower tap is removed and the excess binder allowed to drain off while the silica is restrained. Air at approximately two atmospheres is passed through the compacted cartridge for approximately minutes and the cartridge is placed in an over and heated for three hours at 70C to drive off most of the water. The drying is then completed by raising the oven temperature to 150C and maintaining the same for approximately ten hours, although times of one to twelve hours are satisfactory and longer times may give an improved dryness characteristic. After heating, the taps are closed and the cartridge fuse construction is substantially complete. A pair of cartridges having endcaps 117 are then welded between busses l3 and 114 to provide a twocartridge parallel array having a total of 44 fusible elements therein. The length of each fusible element is approximately 15 inches.

FIG. 4 of the drawing is a schematic plot representing the voltage and current vs. time characteristics of two identical current-limiting fuses illustrating the advantages of the present invention. In FIG. 4, curve A represents a test over-current applied to the fuses and is essentially a simulated dead-short rising sharply from zero current at T 0 and represents the available current. Both fuses tested were 8 inches long and had a cartridge diameter of 1 inch and utilized a single ribbon of silver having a thickness of 0.003 inch and a width of0.0188 inch, each containing 16 holes of 0.0125 inch diameter. These fuses were both rated to sustain a steady-state current of 10 amperes at 5000 volts. Curves B and C, respectively, represent the voltage and current characteristics after fusion of a fuse constructed in accord with the present invention utilizing silica sand bound with colloidal silica. Curves D and E represent respectively voltage and current characteristics of a prior art type fuse utilizing compacted unbound silica sand. As may be seen from FIG. 4, in both instances the fault current is initiated at a time t 0 and both fuses melt at a time t, which is 0.003 seconds from t At that time, the voltage across both fuses is zero and the current is approximately 1100 amperes. From a comparison of curves B and D it may be seen that curve B, the voltage curve for the fuse in accord with the present invention, rises much more rapidly and achieves a much higher peak voltage than does curve D which represents the voltage characteristics of the prior art fuse. This rapidly rising voltage developed across the fuse of the present invention results in an immediate and progressive decrease in the current through the fuse of the present invention until current interruption at a time t which is 0.01 second after initiation of the fault current. Curve E, on the other hand,

indicates that with the fuse of the prior art, due to the slow rise in voltage thereacross and the relatively low value of the peak voltage developed, the current through the fuse progressively increases from a fusion to a higher value before current falls off and reaches zero at essentially the same time as with the fuse of the present invention, namely, at t 0.01 second after the application of the fault current. Thus, FIG. 4 shows graphically that equipment protected by fuses in accord with the present invention is much more protected from high currents. Since the amount of energy allowed to pass to the associated equipment upon fusion of the fuse is represented by the I T term which is obtained by taking the integral of the square of the current I as a function of time from t, to 1 Thus It may readily be seen, therefore, that a very substantial amount of energy derived by the differences of the values of curves C and D is dissipated in circuits fused with fuses of the prior art, but is not dissipated in circuits fused with fuses of the present invention. While the curves of FIG. 4 are intended to show the type of advantage in current and voltage characteristics which may be achieved with fuses in accord with the present invention over fuses of the prior art, the curves of FIG. 4 are illustrative only of one controlled set of tests and are by no means indicative of the greatest advantage in voltage and current characteristics which may be obtained in accord with the present invention.

In summary, there has been disclosed herein a unique new structure for and method of making cartridge fuses of the current limiting type having a particulate arcconstricting media surrounding one or more fusible elements contained within an insulating cartridge. In accord with the invention, a high purity refractory insula tor of high dielectric strength is used as the particulate or pulverent matter, and an inorganic binder, preferably a colloidal suspension of a highly purified, refractory oxide or nitride is used to bind the individual particles of the particulate or pulverent material so as to form a rigid, self-supporting matrix of particulate or pulverent material wherein the particulate material is rendered immobile so that, under the high pressures of an electrically conducting arc which is highly constricted by the arc-constricting medium, the particles do not move to cause a spreading out of the arc and a loss of the high voltage gradient thereof.

The material chosen for the binder is inorganic, insoluble in the material in which it is suspended, and is prepared in the form of a colloidal suspension. Chemically, the material is one which does not decompose under the influence of the high temperature of the arc struck upon fusing of the fusible element of the fuse so as to maintain the immobility of the particles of the arcconstricting medium. Another chemical characteristic of the material is that it does not change in composition, nor decompose, to form any material which is electrically conductive or even highly resistive so as to cause the surrounding binding material to become electrically conductive and degrade the insulating characteristic of the fuse upon extinction of the current carrying arc. Preferably the binder material in colloidal suspension is a refractory oxide or nitride of metals such as silicon, magnesium, aluminum, beryllium, calcium, strontium, and the like.

While the invention has been disclosed herein with respect to certain specific embodiments and examples thereof, many modifications and changes will occur to those skilled in the art. Thus, for example, filler materials other than the oxides and nitrides of the metals specified may be used. Other suitable fillers may include, for example, certain borides and carbides as well. Accordingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true sprit and scope of the foregoing disclosure.

What is claimed is:

1. A current limiting fuse comprising a. an electrically insulating cartridge;

b. a pair of electrically conductive terminals at opposed ends of said cartridge;

c. a fusible element within said cartridge in electrical contact with said terminals;

(1'. a filling of packed particulate matter of high electrical resistivity and high dielectric strength within said cartridge surrounding and providing a porous enclosure for said fusible element,

d,. at least that portion of said packed particulate matter immediately surrounding said fusible element being bound into a rigid self-supporting adhering body by a silica binder added thereto,

d said binder being present in sufficient quantity to substantially coat the entire surface of each particle of said particulate matter, but insufficient to substantially impair the porosity thereof,

d said binder having an electrical resistivity characteristic which does not degrade the electrical resistivity characteristic of said packed particulate matter at temperature up to the temperature of said matter during arcing after fusion of said fusible element and which maintains its binding characteristic at said temperatures.

2. The fuse of claim 1 wherein said particulate matter is characterized by a density at least percent greater than the density of a gravity filled cartridge.

3. The fuse of claim 1 wherein said particulate matter is selected from the group consisting of the oxides and nitrides of silicon, magnesium, aluminum, beryllium, calcium, and strontium.

4. The fuse of claim 1 wherein said particulate matter is silica sand.

5. A current limiting fuse having a rating of at at least 1000 volts and comprising:

a. at least one electrically insulating cartridge having a minimum cross-sectional dimension of approximately one inch; b. a pair of electrically conductive terminals at opposed ends of said cartridge; c. at least one fusible element within said cartridge in electrical contact with said terminals; d. a filling of packed particulate matter of high electrical resistivity and high dielectric strength within said cartridge surrounding and in intimate contact with said fusible element and providing a porous arc-constricting enclosure therefor, d said packed particulate being bound into a rigid self supporting adherent body by a silica binder added thereto,

d said binder being present in sufficient quantity to substantially coat the entire surface of each particle of said particulate matter, but insufficient to substantially impair the porosity thereof,

d said binder having an electrical resistivity characteristic which does not degrade the electrical resistivity characteristic of said packed particulate matter at temperatures up to the temperature of said matter during arcing after fusion of said fusible element and which maintains its binding characteristic at said temperatures.

6. The fuse of claim 5 wherein a plurality of fusible elements are connected between said terminals and individually surrounded by said bound particulate matter.

7. The fuse of claim 5 wherein a plurality of said cartridges are connected electrically in parallel.

8. The fuse of claim 7 wherein the diameter of the cylinders of said cartridges is the same.

9. The fuse of claim 5 wherein said fuse cartridge diameters are within the range of approximately 1 to 4 inches.

10. The fuse of claim 5 wherein said particulate matter is selected from the group consisting of the oxides and nitrides of silicon, magnesium, aluminum, beryllium, calcium, and strontium. 

2. The fuse of claim 1 wherein said particulate matter is characterized by a density at least 10 percent greater than the density of a gravity filled cartridge.
 3. The fuse of claim 1 wherein said particulate matter is selected from the group consisting of the oxides and nitrides of silicon, magnesium, aluminum, beryllium, calcium, and strontium.
 4. The fuse of claim 1 wherein said particulate matter is silica sand.
 5. A current limiting fuse having a rating of at at least 1000 volts and comprising: a. at least one electrically insulating cartridge having a minimum cross-sectional dimension of approximately one inch; b. a pair of electrically conductive terminals at opposed ends of said cartridge; c. at least one fusible element within said cartridge in electrical contact with said terminals; d. a filling of packed particulate matter of high electrical resistivity and high dielectric strength within said cartridge surrounding and in intimate contact with said fusible element and providing a porous arc-constricting enclosure therefor, d1. said packed particulate being bound into a rigid self supporting adherent body by a silica binder added thereto, d2. said binder being present in sufficient quantity to substantially coat the entire surface of each particle of said particulate matter, but insufficient to substantially impair the porosity thereof, d3. said binder having an electrical resistivity characteristic which does not degrade the electrical resistivity characteristic of said packed particulate matter at temperatures up to the temperature of said matter during arcing after fusion of said fusible element and which maintains its binding characteristic at said temperatures.
 6. The fuse of claim 5 wherein a plurality of fusible elements are connected between said terminals and individually surrounded by said bound particulate matter.
 7. The fuse of claim 5 wherein a plurality of said cartridges are connected electrically in parallel.
 8. The fuse of claim 7 wherein the diameter of the cylinders of said cartridges is the same.
 9. The fuse of claim 5 wherein said fuse cartridge diameters are within the range of approximately 1 to 4 inches.
 10. The fuse of claim 5 wherein said particulate matter is selected from the group consisting of the oxides and nitrides of silicon, magnesium, aluminum, beryllium, calcium, and strontium. 